1. Field of the Technology
The present technology relates to a developing device and an image forming apparatus.
2. Description of the Related Art
A copier, a printer, a facsimile or the like is provided with an image forming apparatus which forms an image by means of electrophotography. The electrophotographic image forming apparatus forms an image by forming an electrostatic latent image on a surface of a photoreceptor drum by a charging device and an exposure device, developing the electrostatic latent image by supplying a toner with the developing device, transferring a toner image on the photoreceptor drum to a recording medium such as a recoding sheet by a transfer section, and fixing the toner image on the recording sheet with a fixing device.
The toner supplied to the photoreceptor drum by the developing device is stored in a developer tank provided in the developing device as a developer. The developer stored in the developer tank is conveyed to a developing roller provided in the developing device. The developing roller bears the developer on a surface thereof and rotates so that the developer is conveyed to a vicinity of the photoreceptor drum. The toner contained in the developer is frictionally charged in a process of being conveyed to the developing roller, and the charged toner is moved onto the photoreceptor drum from the surface of the developing roller with an electrostatic force generated with an electrostatic latent image on a surface of the photoreceptor drum. In this way, the developing device develops the electrostatic latent image on the surface of the photoreceptor drum to form the toner image.
As a type of conveying a developer to a developing roller in a developing device, a circulation type has been employed conventionally. A developing device of circulation type has a partition, provided in a developer tank thereof, for partitioning an inside of the developer tank into a conveyance path facing a developing roller, another conveyance path opposed to the conveyance path with the partition interposed therebetween, and two communicating paths communicating with the two conveyance paths at both ends in a longitudinal direction thereof, and in each of the conveyance paths, a developer conveying member is provided for conveying a developer in an opposite direction to each other. Then, by the two developer conveying members, the developer is circulated and conveyed through a first conveyance path, a first communicating path, a second conveyance path, and a second communicating path, in this order.
Japanese Unexamined Patent Publication JP-A 2001-255723 describes a developing device of circulation type with two developer conveying members whose numbers of spirals are different from each other. According to JP-A 2001-255723, differentiating the numbers of the spirals between the two developer conveying members enables suppression of repelling of the developer in the two developer conveying members, so that the developer can be circulated and conveyed smoothly. Furthermore, Japanese Unexamined Patent Publication JP-A 2009-109741 describes a developing device of circulation type having a developer conveying member provided with a reverse spiral blade with a through hole formed therein for conveying a developer in an opposite direction to a developer conveying direction, downstream in a developer conveying direction on a spiral blade of the developer conveying member. According to JP-A 2009-109741, the reverse spiral blade with the through hole formed therein generates convection of the developer downstream in the developer conveying direction, thereby making it possible to prevent retention of the developer.
Even in the developing device described in JP-A 2001-255723, and even in the developing device described in JP-A 2009-109741, in order to move the developer from the conveyance path to the communicating path to circulate the developer, a pressure is needed to be generated on the developer. That is, in the developing device described in JP-A 2001-255723 generates a pressure on the developer by compressing the developer with the developer conveying member and an inner wall of a developer tank in a conveyance path so that the developer is moved toward a communicating path which lies in a direction in which the pressure on the developer is lowered. Moreover, the developing device described in JP-A 2009-109741 generates a pressure on the developer by compressing the developer with the spiral blade of the developer conveying member and the reverse spiral blade of the developer conveying member in a conveyance path so that the developer is moved toward a communicating path which lies in a direction in which the pressure on the developer is lowered.
In a developing device for moving the developer to the communicating path by generating the pressure on the developer in this way, there is a problem that a load to the developer is great so that degradation of an image quality is caused. For example, in a case where a toner contained in the developer is a toner externally added with a fluidity improvement agent, when the toner is compressed excessively by the developer conveying member, the fluidity improvement agent is immersed into a surface of the toner and fluidity of the toner is reduced extremely so that conveyance and sufficient charging of the toner are difficult. As a result, a sufficient amount of the toner cannot be supplied to the photoreceptor drum, thus lowering an image concentration of a formed image.